In the hot pressing of synthetic resin foils, sheets or like members, it is a common practice to interpose between the deformable synthetic resin material and a pressure plate, which may be the hydraulically movable bed or head of the press, a heatable and/or coolable plate, hereinafter referred to as the temperature-controlled or press plate.
The latter plate may be raised to a temperature equal to or above the softening point of the synthetic resin by circulating a heating fluid therethrough, or can be electrically heated. In the application described it is also desirable that the temperature-controlled plate be cooled while the pressure is applied or thereafter to harden the workpiece. The cooling can also be effected by passing a fluid through passages in the temperature-controlled plate.
In other hot-pressing applications, e.g. in the production of pressed board or for laminating purposes, either heating alone or a combination of heating and cooling can be used, the temperature-controlled plate serving for this purpose.
It is known to limit the heat flow to and from the temperature-controlled plate and from the adjoining pressure plate or into the latter by interposing between the pressure plate a plurality of spacers which provide thermal insulation and act as force-transmitting members allowing the press pressure to be applied to the temperature-controlled plate and then the material to be compressed.
These spacers have a dual function, therefore, in that they not only limit heat flow between the pressure plate and the temperature-controlled plate, while acting as force-transmitting members, but they permit dimensional change because of thermal phenomena, e.g. expansion and contraction of the temperature-controlled plate with minimum wear of contact surfaces between the temperature-controlled plate and the pressure plate.
Without such wear reduction, the damage to the press plate is usually so pronounced that the operating life of the press is severely limited.
Furthermore, the spacers minimize the distortion of the press plates in the closed state of the press, i.e. when the press pressure is effective and thereby prevent damage to the workpiece which is especially important in the case of shaping plastic members.
The spacers have been provided in various configurations heretofore and reference may be made especially to the German patent document (Offenlegungsschrift) DE-OS No. 2,354,281 in which the spacers are massive bodies of filled polyamide with a height/diameter ratio less than 1. These spacers are provided in openings of a shield or barrier of thermally insulating material, the thickness of this layer being less than the height of the spacer.
The shield or barrier layer can be composed of wood cement. These constructions have been found to be satisfactory only for relatively small capacity presses, i.e. presses whose play area is limited. In such presses the movement of the temperature-controlled plate relative to the pressure plate is comparatively small.
For large-format presses, however, in which the relative displacement of the two plates because of their temperature difference is relatively high these comparatively large movements can result in entrainment of the spacers, distortion of the shield and nonuniform distribution of force over the surface of the temperature-controlled plate. Because of the nonuniform force distribution at high-plate pressures, plate deformation can occur with obvious production disadvantages and possible permanent damage to the press members.
To avoid these disadvantages it is not uncommon for the operator to reposition the spacers between press operations at considerable labor expense by time-consuming procedures.